Using grism to find faint dwarf galaxies furiously forming stars

This video shows how astronomers can use the NASA/ESA Hubble Space Telescope’s highly-sensitive Wide Field Camera 3 in its grism spectroscopy mode to find bursts of star formation in small and distant galaxies.

A grism is a combination of a grating and a prism, and it splits up the light from a galaxy into its constituent colours, producing a spectrum. In this video the continuum of each galaxy is shown as a "rainbow".

Astronomers can look at a galaxy’s spectrum and identify light emitted by the hydrogen gas in the galaxy. If there are stars being formed in the galaxy then the intense radiation from the newborn stars heats up the hydrogen gas and makes it glow.

All of the light from the hydrogen gas is emitted in a small number of very narrow and bright emission lines. For dwarf galaxies in the early Universe the emission lines are much easier to detect than the faint, almost invisible, continuum.